Turbine engines, such as single shaft industrial gas turbines, are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load. Any reduction from the maximum possible base load power is referred to as part load operation. In other words, part load entails all engine operation from 0% to 99.9% of base load power.
Part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, mass flow of air through a turbine engine 10 (FIG. 1) can be restricted by closing the compressor inlet guide vanes (IGV) (not shown), which act as a throttle at the inlet of the compressor 12. When the IGVs are closed, the trailing edges of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing throat area reduces the flow of air which the first row of rotating blades can draw into the compressor 12. Lower flow to the compressor 12 leads to a lower compressor pressure ratio being established by the turbine section 11 of the engine 10. Consequently, the compressor exit temperature decreases because the compressor 12 does not input as much energy into the incoming air.
Some of the compressor exit air 14 from the combustor shell 15 is used to cool the stationary support structure 16 of the turbine near the first row of blades 20a. The stationary support structure 16 can include the outer casing, blade rings, and ring segments. In addition, some compressed air is piped directly out of the compressor 12 through piping 19a (additional pipes not shown). This compressor bleed air is used to cool the stationary support structure 16 near the second, third and fourth rows of blades 20b,20c,20d and is supplied through piping 19b,19c,19d. Because of the decrease in compressor exit and bleed air temperature due to the closed IGV position, the support structure 16 will contract, that is, it will shrink in radius when exposed to the cooler compressor exit and bleed air. But, at the same time, the temperature of the hot gas leaving the combustor 18 and flowing over the turbine blades 20a,20b,20c,20d (hereafter collectively referred to as “20”) is kept at a high level, causing a constant radially outward thermal expansion of the blades 20.
The expansion of the blades 20 along with the shrinkage of the support structure 16 reduces the clearance C between the tips 21 of the blades 20 and the surrounding support structure 16, commonly referred to as the blade tip clearance C. While the clearance C is shown between the fourth row of blades 20d and the adjacent support structure 16, similar clearances C exist between the first, second and third rows of blades 20a,20b,20c and the stationary support structure 16. It is critical to maintain a minimal blade tip clearance C sufficient enough that the blades 20 do not rub against the support structure 16; however, this constraint limits the load reduction which can be achieved with the combustor exit temperature kept near that of base load temperature.